1. Field of the Invention
The invention relates to a Francis runner and to a hydraulic machine equipped with such a runner.
Francis runners can equip different sorts of hydraulic machines, such as turbines, pumps, or pump-turbines. They comprise blades distributed about a central rotating shaft and define therebetween channels for flow of water. The geometry of the blades of these runners is defined so that the flow of the water induces a torque on the runner, in the case of a turbine, or so to transmit a movement to the fluid, in the case of a pump. The power that a hydraulic machine equipped with such a runner can deliver depends on its geometry and on the type of heads with which it is associated. In this way, the power that a turbine can deliver may be brought to a reference value defined by the equivalent power delivered by a turbine of the same geometry working under 1 metre of head and whose runner outlet diameter is 1 metre. This power P11 depends in particular on the speed of rotation N11 of the turbine under the same conditions.
2. Description of the Related Art
As is visible in FIG. 6, an optimal working point A may be defined in a system of coordinates giving the power P11 of a turbine, under the aforementioned conditions, as a function of the speed of rotation N11 under the same conditions. There is defined as power under high load P11FC, the power of the turbine for an efficiency less by 3.5% than the efficiency at point A. In the reference system P11 on N11, curves I99, I98, I97, etc. of constant values of the efficiency obtained with a turbine are defined. Furthermore, there is defined a noteworthy point B of the same abscissa as point A and for which the power obtained is equal to P11FC.
There is defined as equivalent power under high load P11FC, the power obtained under the conditions of point B for each turbine.
As shown in FIG. 7, present-day turbines have equivalent powers under high loads P11FC which, in a representation as a function of the speed N11 mentioned hereinabove, lie in a first zone Z1, which shows that the equivalent power under high load P11FC increases as a function of the speed N11. It is sometimes necessary to obtain relatively high equivalent powers under high loads. In particular, in the case of rehabilitation of an existing installation, the speed N11 is imposed, this in practice limiting the power values P11FC that may be obtained with a conventional turbine.
Up to the present time, equivalent power zones under high loads of relatively high values with respect to the speed N11 have not been really explored by the designers of hydraulic machines, as solutions degraded from the technical/economical standpoint were expected.